1. Technical Field
The present invention relates to an image display method, an image display processing program, and an image display apparatus capable of performing color sequential display.
2. Related Art
Color image data is represented as a collection of a small number of color signals. In most cases, the data is represented using three primary color data of red (R), green (G) and blue (B) as reference colors. For actual display, cyan (C), magenta (M) and yellow (Y) as complementary colors of the reference colors R, G and B and other colors are used in some cases.
For displaying color images using these plural color signals, there are currently a color simultaneous display method which displays all color signals simultaneously, and a color sequential display method which displays respective color signals in a time-sequential manner.
In the color simultaneous display method, the respective colors are placed side by side or overlapped with one another in a space to be simultaneously displayed. This method provides natural display, but requires a complicated display apparatus. In a liquid crystal display panel or a plasma display, for example, respective R, G and B display elements need to be arranged extremely minutely. In the case of a projection-type display apparatus such as a projector, three light modulation elements (called electro-optical modulation devices) for producing respective R, G and B images and a synthesizing section for accurately overlapping these three single-color images into one full-color image need to be equipped.
On the other hand, the color sequential display method disposes the respective colors in a time-sequential manner for sequential display. Since only a single color image is displayed at a certain moment, only one electro-optical modulation device is required. Thus, the display apparatus can be relatively simplified and miniaturized.
The color sequential display method can be practiced because the human vision has an integral characteristic for the time constant of several ten seconds. More specifically, images sequentially displayed within this time constant are recognized not as separate images but as images of mixed colors to the human eyes, and the color sequential display method utilizes this characteristic. Thus, when the respective R, G and B images are switched to one another at high speed during display, these images are recognized as full-color images produced by the three color signals R, G and B.
Accordingly, the color sequential display method is advantageous in view of simplification and miniaturization of the system and cost reduction compared with the color simultaneous display method. However, in exchange for the advantages, the color sequential display method has drawbacks such as color splits in display and a problem of response performance of the electro-optical modulation device, which do not occur in the color simultaneous display method.
Color splits in display are caused when the respective images are not accurately overlapped with one another on the retina in the color sequential display method. Thus, the color sequential display method is practiced on the assumption that the single-color images arranged in the time-sequential manner are accurately overlapped on the retina.
However, when the vision is shifted during the color sequential display, i.e., when images on the retina are moved to another position, the images are not correctly overlapped and thus the respective color signals of the original images are separately recognized. This phenomenon is called color splits, which cause severe deterioration in image quality.
The problem of response performance of the electro-optical modulation device is caused due to shortened display time of the single-color images resulting from time-sequential arrangement of the images.
More specifically, a digital-type electro-optical modulating device such as a digital mirror device (DMD: registered trademark) requires a certain period of display time to be secured since it shows gradations by such a method as pulse width modulation. For highly accurate display within a short display time, the DMD needs to perform high-speed processing, which leads to higher power consumption, shorter life, unnecessary electromagnetic radiation and other problems.
Additionally, an analog-type electro-optical modulation device such as a liquid crystal display element requires a certain time period for obtaining outputs in correspondence with inputs.
FIG. 25 schematically shows a thin film transistor (TFT) type liquid crystal display device as an example of the electro-optical modulation device. The TFT type liquid crystal display device has TFTs 101 provided on a board by a thin film technique, liquid crystal cells 102, and retention capacitors 103 in correspondence with respective display elements, and signal lines (gate lines 104 and source lines 105) through which signals are supplied. As illustrated in the figure, the liquid crystal cells 102 are considered as capacitors from the viewpoint of electric circuit. Thus, the conditions of the liquid crystals vary in accordance with the voltage charged to the liquid crystal cells 102, thereby changing the polarization conditions of light which passes through the liquid crystal cells 102 for display.
The light modulation outputs from the liquid crystal display device are variable in accordance with the voltage charged to the capacitors (liquid crystal cells 102). Thus, display response delay is caused in correspondence with delay of charge to the capacitors. More specifically, the response time of the currently used high-speed liquid crystal display device is as long as several milliseconds. This time period is almost equal to 6 milliseconds as the display time of 180 sub frames per second when images of 60 frames per second are displayed in the time-sequential manner using the three primary colors of R, G and B, for example. Therefore, the response time cannot be ignored at all.
A related art which reduces color splits and solves the problem of response time has been disclosed in JP-A-8-248381. According to this technique, the color order of R, G and B for color sequential display is switched on the cycle of three frames, such as RGB, BRG, and GRB, as shown in FIG. 16 and other figures attached to the specification of the reference.
FIG. 26 shows a color order for color sequential display according to JP-A-8-248381. As illustrated in FIG. 26, colors can be switched by turning on and off the respective R, G and B light sources in this method. Alternatively, colors can be switched using a color wheel divided into 9 parts. As apparent from FIG. 26, lights emitted from the respective light sources R, G and B do not overlap with one another at any time in the color order of the method according to JP-A-8-248381.
Since the color order shown in FIG. 26 is used in the method of JP-A-8-248381, the same colors are displayed on the boundary between the adjoining frames. As a result, the display time of these colors is increased to twice. Accordingly, the electro-optical modulation device obtains longer response time. Additionally, the colors of R, G and B are equally disposed in the respective sub-frames on the three-frame cycle in this method. As a result, it is expected that R, G and B can be equally mixed in an image having color splits and that deterioration in image quality due to color splits can be thus reduced.
JP-A-2003-280614 is another related art which gives particular attention to the reduction of color splits. According to this technique, the six color system constituted by not only the three primary colors of R, G and B but also cyan (C), magenta (M) and yellow (Y) as complementary colors of R, G and B is used for display.
FIG. 27 shows the color order in color sequential display according to the method disclosed in JP-A-2003-280614. In this method, each frame is divided into 6 sub frames, and colors of Y, B, M, G, C and R are allocated to the respective sub frames one for each. These six colors can be provided using the three R, G and B light sources. In this case, the complementary colors C, M and Y in the sub frames corresponding to the complementary colors C, M and Y can be obtained by using lights emitted from the two light sources. According to JP-A-2003-280614, it is recommended that the intensity of light emitted from the respective light sources in the sub frames of complementary colors is reduced to half so that the overall intensity of light in the sub frames corresponding to the reference colors R, G and B and in the sub frames corresponding to the complementary colors C, M and Y can be equalized.
According to the method disclosed in JP-A-2003-280614, therefore, the width of color splits can be reduced by using the six color system for display, and image quality can be improved by mixing primary colors and complementary colors disposed adjacent thereto in a space in the color order shown in FIG. 27 for display.
Additionally, according to the description in JP-A-2003-280614, the complementary colors are disposed at the same sub frame positions for every two frames. This positioning allows mixing of colors in consideration of time as well as in consideration of space.
According to the technique disclosed in JP-A-8-248381, color splits cannot be sufficiently reduced and the problem of response performance (response delay) cannot be sufficiently solved. More specifically, even if the colors of R, G and B are equally disposed at the same sub frame positions on the three-frame cycle in the method of JP-A-8-248381, this cycle is too long considering the time constant for the human vision characteristic. This problem is clarified as a problem to be solved in the method of JP-A-8-248381 in the specification of JP-A-2003-280614. Since the three-frame cycle of image data having 60 frames per second is 20 cycles per second, one cycle is 50 milliseconds. This period is too long compared with the time constant for the human vision.
As for the problem of response time in the method of JP-A-8-248381, the same color display continues only on the boundary of the frames as illustrated in FIG. 26. Thus, the effect of successive display of the same color is not given to other parts.
On the other hand, the method disclosed in JP-A-2003-280614 can reduce color splits to a significant extent. However, according to this technique, a larger problem concerning response delay of the electro-optical modulation device than the problem in the method of JP-A-8-248381 may be caused. Since one frame is constituted by six sub frames in the method of JP-A-2003-280614, the display time of one sub frame is half of that in the method of JP-A-8-248381 which has three sub frames in one frame. This is not a serious problem if the response time of the electro-optical modulation device is sufficiently short in accordance with this time reduction. However, in the currently used electro-optical modulation device, this requirement is not sufficiently satisfied.
Therefore, both techniques disclosed in JP-A-8-248381 and JP-A-2003-280614 are not optimum methods which can sufficiently solve the problems particularly concerning the response performance of the electro-optical modulation device.